1

2

Surfactants in Ink-Jet Inks⊗ H-bond

Boris Zhmud and Fredrik Tiberg 1. Introduction This chapter gives a brief overview of the use of surfactants in inks for ink-jet printing. The purpose is to provide the reader with a short, self-consistent, and let's hope, pedagogically balanced introduction into this highly technical but at the same time still largely empirical subject. For reasons of both intellectual property and, more importantly, clarity, specific formulation issues are only touched upon; instead a major emphasis is made on general aspects of surfactant action that, in our opinion, explain the role played by surfactants in inks. Both ink production and various stages of the ink-jet printing process are discussed in this context. Viewing the wetting enhancement as a primary function of surfactants, effects of surfactants on ink spreading and penetration are covered in more detail. A large body of pertinent technical information in patent and technical literature is overviewed and systematized, in order to provide the reader with a broad survey of how these principles are implemented in practice.

FIGURE 1 Hydrogen bond NO. . .HO between a water molecule and the NO group in N-alkyl dimethyl amine oxide surfactants [5].

2. Properties of surfactants The term "surfactant" normally refers to water-soluble organic compounds whose molecules have a bipolar structure with hydrophilic and hydrophobic moieties localized at the opposite ends. This renders surfactant molecules amphiphilic in a sense that the hydrophilic terminal has a higher affinity to polar solvents, such as water, and is pulling the molecule towards the polar phase, while the hydrophobic terminal has a higher affinity to non-polar solvents, such as oil, and is pulling the molecule towards the non-polar phase [1-4]. For that reason, the word "hydrophobic" is sometimes replaced by "lipophilic". For example, when such surfactant as N,N-dimethyl-N-(ndodecyl)amine oxide (DDAO),

Me

+ O N Me

is added to an aqueous solution, the polar heads of DDAO molecules get hydrated leading to a release of about 9.7 kcal/mol of energy [5]. The electron density map of the hydration complex shows that there exists strong hydrogen bonding between the NO group and water (see Figure 1). At the same time, the aliphatic tail has no affinity to water - in fact, its hydration is energetically unfavourable because the cavitation energy is so high that it cannot be compensated by the energy of weak interactions between water and the non-polar aliphatic tail. For that reason, the aliphatic tail prefers a non-polar phase, such as oil or air.

⊗

Published in "Surfactants in Polymers, Coatings, Inks and Adhesives." (D.R.Karsa, Ed.), Blackwell Publishing, England, 2003, chap.8.

As a result of such an amphipathy for both polar and non-polar phases, surfactant molecules can only make themselves snug at the interface between the polar and non-polar regions. As many molecules as possible try to stuff up the interface creating a surface excess of surfactant - the surfactant is said to be adsorbed to the interface - and a surface pressure, π, which is a function of the amount of surfactant adsorbed. The surface tension, γ, of the interface is reduced as compared to the surface tension of the interface free of surfactant, γ0, g =g 0 -p (1) The above-mentioned facts are formalized in the Gibbs equation, c

c

0

0

p = g 0 - g (c) = ò Gdm =RT ò Gd ln c

(2)

where µ denotes the chemical potential, and Γ, the surface excess, of surfactant; Γ is obviously an ascending function of the bulk concentration, c, of surfactant. The dependence Γ(c) is often described by the Langmuir adsorption isotherm, G K c (3) G (c ) = m L 1 + KLc where Γm is the monolayer capacity expressing the maximum amount of surfactant that can be accommodated at the interface, and KL is the Langmuir adsorption constant. Substituting eq.(3) into eq.(2), one gets the Langmuir-Szyszkowski equation, g (c) = g 0 - G m RT ln(1 + K L c) (4) showing a decrease in the surface tension brought about by surfactant adsorption. A more general type of the adsorption isotherm is

é A E + wf ö ù æ f = ê1 + exp çy − ÷ RT ø úû è ë c

−1

(5)

3

4

where f = G / G m , E is the adsorption energy, ω is the energy of lateral interactions, and A is a parameter. By putting y = 0 , one gets the Langmuir, Frumkin, and Fowler-Guggenheim isotherms,1 whereas putting y = f /(1 − f ) leads to the Hill-de-Boer isotherm. As c → 0 , all the

c(∞, t ) = C0 (11) the first of which shows that all the surfactant transported through the subsurface is accumulated at the liquid/vapour creating a surface excess, Γ(t), of surfactant at the latter, and the second says that the bulk concentration of surfactant is not affected by adsorption. From the last two conditions, one gets t ∞ é ∂c ù é ∂ æ ∂c ö ù æ ∂c ö G (t ) = ò dt ê D (12) ú = ò dt ò dx ê ç D ÷ ú = ò dx ò dt ç ÷ = ò {C0 − c( x, t )}dx x x x t ∂ ∂ ∂ ∂ è ø è ø ë û x =0 û ë 0 0 which accounts for the mass conservation law. The surface excess of surfactant at the liquid/vapour can be related to the concentration of surfactant on the subsurface, (13) G (t ) = F o c(0, t ) hence ∞ ïì ïü c(0, t ) = F −1 o í ò [C0 − c( x, t )]dx ý (14) ïî 0 ïþ -1 where F(c) describes the adsorption isotherm, and F represents the inverse function of F. In the linear approximation, G (t ) = K H c(0, t ) , and hence

Gm æ E ö is the Henry adsorption exp ç è RT ÷ø A constant. However, from a practical viewpoint, the generalized Frumkin isotherm, KF c f (c ) = (6) exp( pf q ) + K F c where KF is the Frumkin adsorption constant and p and q are two additional fitting parameters, is often found to be the best one when it comes to fitting experimental adsorption data. If the lateral interaction are relatively weak, so that |p| 90o (non-wettable substrate)

cmc

10-3

10-2 R

-3

concentration [mol dm ] FIGURE 5 Determination of cmc for CnE6 (n = 10,12,14) surfactants by surface tension titration (after Zhmud et al. [6]). There exists a continuous exchange of surfactant molecules between the solution and the micelles. The average lifetime of a molecule in a micelle is about 10-5 s. For each surfactant, there exists a certain critical temperature at which the solubility increases abruptly. This temperature is called the Krafft point. Below the Krafft point, the surfactant may be practically insoluble in water, and its surface activity is very low. However, above the Krafft point, the apparent solubility of surfactant increases tremendously due to micellization, and so does the surface activity [2]. Micelles can also encapsulate a substantial amount of other compounds which otherwise would be insoluble in water. This effect is called solubilization [3]. In water-based ink formulations, surfactants can be used to solubilize dyes.

θ

γlv

rb

γsv

γsl

The gravity effects can be neglected because of a small size of the drop. There are three interfaces contributing to the total excess free energy of the system shown,5 E = Elv + Esl + Esv (28) where Elv = g lv Slv = 2p R 2g lv (1 − cosq ) Esl = g sl S sl = p R 2g sl sin 2 q

(29)

Esv = g sv S sv = const − p R 2g sv sin 2 q

5. The theory of surfactant-induced wetting The wetting ability of a liquid with respect to a solid surface can be characterized by measuring the contact angle between by the liquid meniscus and the surface; a contact angle less than 90o indicates that the substrate is readily wetted by the test liquid, while an angle greater than 90o shows that the substrate will resist wetting.

The value of θ corresponding to the minimum E is found from the equation, dE =0 dq

(30)

5 There exists an additional contribution, Esvl, from the three-phase contact line where all the three interfaces intersect. However, the corresponding correction is only significant for sub-micron size drops. In ink-jet printing, the typical drop size is about 10 µm, hence the line tension effects can be safely ignored.

11

12

taking into account that the volume, V, of the drop is constant and the radius, R, of the drop is related to the contact angle, 3V (31) R (q ) = 3 p (2 - 3cosq + cos3 q ) Then, after some basic arithmetics, one arrives at eq.(27). Furthermore, the wetting force acting per unit length of drop perimeter is given by 1 ¶E 1 ( ¶E / ¶q ) == g lv (cosq - cosq ') (32) F (q ') = rb ¶rb rb (¶rb / ¶q ) where q is the equilibrium contact angle determined by the Young-Dupré equation, q ' is the out-of-equilibrium contact angle, and rb (q ') = R (q ')sin q ' is the radius of the drop base corresponding to a given value of q ' . It is clear that high energy substrates, which are characterized by a high value of γsv, will be readily wettable by most liquids. In contrast, low-energy substrates are only wettable by liquids whose own surface tension, γlv, is low enough. Since surfactants have been shown to reduce γlv, and probably γsl as well, they promote wetting. The equilibrium contact angle between a solid surface and a drop of a surfactant solution is a function of surfactant concentration, herewith

A few notes regarding applicability of the Young-Dupré equation should be stated. First, if γsv - γsl > γlv, there is no real contact angle that would meet the equation. In this case, referred to as the complete wetting, the liquid will spread to a monolayer film. Second, if γlv is always positive, γsv and γsl can in general be negative. In the latter case, the stability of the corresponding interfaces is ensured by mechanical hardness of the solid substrate. Although there is no direct methods for determination of the surface tension of solids, it is believed that many superspreaders can render the surface of the sl interface negative. The wettability can also be characterized by the spreading coefficient, S = γsv - γsl - γlv (34) positive values of S corresponding to the complete wetting. For γsv - γsl < γlv, the value of S an be found from the contact angle, S = g lv (cosq - 1) (35) and normally is negative. Let a liquid drop be deposited at a solid surface and start to spread. Since all liquids are characterized by certain equilibrium vapour pressure, i.e., at a given temperature, there always exists a certain amount of vapour in equilibrium with condensate, and diffusion of molecules in the gas phase is quite rapid, it seems to be quite logical to assume that there should be a thin precursor film advancing in front of the spreading liquid. In other words, some amount of liquid vapour adsorbs to the solid surface before that surface is covered by the expanding drop. By analogy with eq.(2), such a pre-wetting of the solid surface brings about a decrease in γsv [9],

cosq (c) =

g sv - g sl (c) g sv - g sl (0) + RT ò G sl (c)d ln c = g lv (c) g lv (0) - RT ò G lv (c)d ln c

p (T )

(33)

ì ü RT = cosq (0) í1 + ò [G sl (c) + Glv (c)]d ln c + ...ýþ > cosq (0) î g lv (0) i.e. the addition of surfactant leads to a decrease in the contact angle, q (c) < q (0) . This is a thermodynamic interpretation of the wetting-enhancing effect of surfactants. It should be noted that, depending on adsorption affinity of surfactant to various interfaces, either wetting enhancement or dewetting effect may prevail. These two scenarios are illustrated in the following drawing:

low γlv

low γlv

g sv ( p) = g sv0 − RT

ò

G ( p )d ln p

(36)

0

where g sv0 is the surface tension of the dry surface, and Γ(p) is the vapour adsorption isotherm. Note that the vapour pressure, p, is a function of temperature, T. If the resulting decrease in γsv is large enough, the authophobic effect can be observed, when the precursor film slows down the spreading. Furthermore, by taking into account that ps æ p ps ö g sv ( ps ) = g sl + g lv = g sv0 − RT ç ò + ò ÷ G ( p)d ln p = g sv ( p) − ò G ( p)d ln p (37) ç ÷ p è 0 p ø where ps is the pressure of saturated vapour, and using the Young equation, one arrives at ps

g lv cosq = g sv ( p) − g sl = g lv + RT ò G ( p)d ln p

(38)

p

Therefore, the condition of complete wetting can be rewritten as ps

d = ò G ( p )d ln p ≥ 0

(39)

p

low γsl

high γsv

Wetting enhancement (γsv > γsl + γlv )

high γsl

low γsv Dewetting (γsv < γsl + γlv)

Dewetting will normally occur if there exists a strong specific interaction between the polar group and the substrate, forcing surfactant molecules to adsorb in a configuration where their hydrophobic tails extend towards the solution phase. The result of such a specific adsorption is that the substrate surface becomes more rather than less hydrophobic.

Neglecting specific intermolecular interactions, such as hydrogen bonding, steric interactions, etc., the magnitude of δ can be evaluated theoretically by using the Lifshitz theory, p kT ∞ d ; 2 å ln(1 + D sl D lv ) l0 n = 0 (40) e i (ix n ) − e j (ix n ) nkT D ij = , xn = e i (ix n ) + e j (ix n ) h where εi(iξn) is the permittivity of the ith phase (i.e. i = s for solid, l for liquid, and v for vapour) at an imaginary frequency iξn, and λ0 is the lower bound of wave lengths allowed for; typically λ0 is of the order of magnitude of a few angstroms. Adsorption of surfactant creates an interlayer

13

14

whose dielectric properties significantly differ from those of the substrate and the solution. Thereby, the presence of surfactant affects the magnitude of dispersive forces acting between the substrate and the solution. This gives an alternative, and perhaps less straightforward, interpretation of the effect of surfactant on the wetting behaviour of solutions [10]. At the present, paper is by far the commonest printing medium. The ink-receptive layer in paper is porous. Pores are needed to enable drainage of ink vehicle after the ink has been transferred to surface of paper. Both coated and uncoated papers are printable, although the coated stock provides superior print quality. Carbonate coatings are most common, although special coatings may be used for photoquality papers. Paper is often sized with sizing components for the purpose of improving waterfastness, dimensional stability and wet strength of paper. Acid sizing chemicals, such as rosin, or alkaline sizing chemicals, such alkenylsuccinic anhydride and alkylketene dimer, are the primary sizing components. However, such sized papers, when used with an ink jet printer containing predominantly water based inks, tend to yield images with pronounced intercolour bleed, which is a common printing fault showing itself as the invasion of one colour into another on the surface of the printed substrate or in the substrate itself. This undesirable effect can be reduced by using a number of surfactants that effectively act as desizing agents by promoting penetration of ink into the paper structure. Due to their wetting-enhancing effect, surfactants are used as wetters in inks designed to print on other low-energy substrates, e.g. plastics. The effect of surfactant on the spreading behaviour of aqueous solutions over the surface of sized paper can be seen in Figure 6.

The majority of inks designed for ink-jet printing on paper dry by absorption.6 Therefore, it is instructive to discuss the influence of surfactant on the dynamics of capillary penetration [1214]. Here, two feasible scenarios are: (i) the surface tension of solvent or solvent/co-solvent mixture used in the ink formulation is sufficiently low, so that no surfactant is needed to promote ink absorption; and (ii) the surface tension of solvent or solvent/co-solvent mixture used in the ink formulation is sufficiently high, so that an appropriate surfactant needs to be included in the ink formulation to promote ink absorption. The first case is common for solvent-based inks; it should be noted, however, that surfactant can be used even though there is no need to assist wetting, for other goals such as pigment dispersing or anti-foaming are often pursued. The second case is typical of water-based compositions, which are organics-free or contain a low fraction of organic co-solvent. When ink drops ejected from the printhead arrive at the surface of paper, their surface tension may still be too high to enable absorption. However, as soon as the surface tension of the ink relaxes so that the contact angle between the ink and the paper surface becomes less than 90o, the capillary forces start to drive the ink into pores of the ink-recording sheet. Let us consider an ink drop deposited at the surface of a porous coating layer. A suitable theoretical model of such a system is a hydrophobic capillary in contact with a sufficiently large reservoir containing a surfactant solution [14]. At the instant the capillary comes in contact with the solution, there exists a surface excess of surfactant at the lv interface. The thickness of the surfactant-rich layer can be defined as +∞

time [s]

h=

0.5 mmol/L solution of C14E6

ò [exp(− bF( z )) − 1]dz

−∞

exp(− bF min ) − 1

, b = (kT ) −1

(41)

where Φ is the adsorption potential; negative values of Φ corresponds to positive adsorption, and Φmin represents the depth of the potential well. The numerator in the above equation represents the net surface excess, while the denominator gives the maximum density of adsorbate in the adsorbed layer. The order of magnitude of h can be estimated as, G h ; lv (42) rMw

contact angle [deg.]

contact angle [deg.]

Pure water

where Γlv is the surface excess, ρ, the density, and Mw, the molecular weight, of surfactant. Typically, h does not exceed a few molecular diameters. Studies on the mechanism of surfactant transport during the capillary rise of surfactant solution suggest that, under certain conditions, surfactant from the surfactant-rich adsorption zone nearby the meniscus re-adsorbs onto the capillary walls as the solution is advancing through the capillary. This can be imaged as the surfactant spillover through the three-phase contact line [14]. The amount of surfactant transferred per unit time from the lv interface to the sl interface is approximately given by, æ k + G ö ïü ïì G sl = G msl í1 − exp ç − slm lv ÷ ý (43) è G sl z ' ø þï îï

time [s]

FIGURE 6 Comparison of the spreading behaviour of drops of pure water with that of a surfactant solution. Alkylketene dimer-sized papers with different size load are used as substrates (after Zhmud et al. [11]).

where Γsl is the amount of surfactant adsorbed to sl interface, G msl is the monolayer capacity for the sl interface, ksl+ is the adsorption rate constant, and v = z' (here ' = d/dt) is the velocity of the advancing meniscus.

6

To be more precise, the ink is said to set up by absorption of ink vehicle into pores, followed by actual drying, which involves removal of absorbed solvent via evaporation over a longer time.

15

16

c = c ( z ,t )

g lv = g lv0 − RT

ò

c =0

G lv (c)d ln c

(46)

put in contact with the surfactant solution, the capillary rise does not start until a positive capillary force, f(t) > 0, builds up. The corresponding delay time can be estimated from eq.(45).

G lv (t )

25 20

10.0 mM 2.0 mM

15

0.5 mM

z(t)

1.0 mM

z(t ) [mm]

Since the lv interface is continuously depleted by transfer of surfactant to the sl interface, a diffusional flux will develop bringing surfactant along the liquid column from deeper zones to the top. This is schematically depicted in Figure 7. In contrast to pure liquids, where the capillary force is essentially constant, in a surfactant solution the latter is allowed to change as the meniscus is advancing through the capillary. Assuming a vertical rise, the following equation can be used to describe the dynamics of the capillary liquid, 2 8h r s [ zz ''+ ( z ') 2 ] = f (t ) − 2 zz '− r gz (44) r r where é æ k + G öù (45) f (t ) = g lv ê1 − 2exp ç − slm lv ÷ ú è G sl z ' ø ûú ëê

bulk reservoir c(0, t ) = C0

and ρs is the density, and η, the viscosity, of the surfactant solution, and g is the acceleration of gravity. Since both The model FIGURE 7 convective and diffusive fluxes contribute to the transport of system for studying the surfactant through the capillary, so that the total upward flux is capillary rise of surfactant ∂c (47) j = cv − D solutions. ∂z The distribution of surfactant along the capillary is determined by the equation, ∂c ∂ 2c ∂c (0 < z < z (t )) (48) = D 2 − z' ∂t ∂z ∂z For a comprehensive theoretical analysis of this problem the reader is referred to another publication [14], although one special type of capillary rise dynamics characteristic of surfactantinduced penetration warrants a brief discussion here. Let the lv interface get strongly depleted because the diffusional flux fails to compensate for surfactant spill-over on the capillary wall. In this case, the diffusion-controlled dynamic regime is entered. Simple estimates show that in most cases the extension of the diffusion zone is much less than z and scales as (Dt)1/2 with time. Hence the concentration gradient is approximately proportional to C0/(Dt)1/2. Within certain time limits, the capillary rise represents a quasi-steady process, viz.: the amount of surfactant adsorbed to the sl interface per unit time is equal to that brought to the lv interface by diffusion, C0 2p r G msl dz ≈ p r 2 D dt (49) ( Dt )1/ 2 which leads to the following scaling relation, rC z (t ) » m0 ( Dt )1/ 2 (50) G sl This explains the nearly linear appearance of the z vs. t1/2 plots within certain time limits (Figure 8). In this connection, it should be stressed that the Lucas-Washburn equation, which also predicts a linear proportionality between z and t1/2 in the short-time limit, is not applicable in this case. Notice that all the curves in Figure 8 exhibit some initial delay: after the capillary has been

10 5 0 0

2

4 1/2

t

6

8

10

1/2

[s ]

FIGURE 8 Capillary rise dynamics observed for C14E6 surfactant solutions of various concentrations in hydrophobic capillaries of 0.1 mm radius. These demonstrate pseudo-Lucas-Washburn dynamic behaviour.

6. Ink-jet printing Ink-jet printing is a non-impact printing technology where a printed image is created by microscopic droplets of inks jetted out from a drop generator and deposited on a printing substrate [15]. The present-day ink-jet technology is sufficiently mature and allows one to produce high quality print at low cost, which, in combination with relatively noise-free operation of ink-jet printers and the ease in customizing digital printing jobs, leads the way to rapid expansion of the sector of ink-jet printing applications. In the past decade, ink jet printing has become extremely popular for home office, small office and personal printer applications. In more recent years, the popularity of ink jet printing has increased even further due to the introduction of whole digital systems offering photographic quality colour and graphics capabilities. As a result of this healthy development, ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.

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18

Ink-jet printers use the continuous jet and the drop-on-demand printing technologies, the latter prevailing in today's ink-jet printing. Since drop-on-demand systems require no ink recovery, charging, or deflection, they are much simpler and cheaper than their continuous stream counterparts. The drop-on-demand methods differ from each other in the way the ink drops are generated: the most popular being thermal and piezoelectric ink-jet pens. The conceptual mechanism of drop generation in a thermal ink-jet pen is shown in Figure 9. This technique is implemented, for example, in Hewlett Packard DeskJet series of printers.

firing resistor

7. Ink formulations Development of ink-jet technology has three main objectives: (i) reliability, (ii) speed, and (iii) quality. Reliability is evaluated with respect to several criteria such as stability of ink-jet printhead operation in continuous and intermittent printing regimes, and robustness of the ink and printhead with respect to long storage or idling times at various ambient conditions. The maximum speed of printing is, among other things, controlled by ink setting time; a quicker setting ink permits faster printing. However, it turns out to be difficult to combine good reliability with high printing speeds, for formulating ink compositions that print reliably and dry rapidly is quite a tricky job. The majority of modern ink-jet inks are complex fluids consisting of a colourant, ink vehicle, and a number of additional components. Main constituents of a typical ink-jet ink formulation and their functions are listed in Table 1.

egress for ink

TABLE 1 Major components of ink-jet inks and their functions

nozzle plate

printed substrate FIGURE 9 Schematic construction of a thermal ink-jet pen When an electrical current pulse is sent through the resistor, the heat evolved causes overheating of the nearby ink to the critical temperature, which is around 280-300oC for water-based formulations [16]. As the ink reaches its critical temperature, bubble nucleation commences. Once nucleated, the bubble thermally isolates the ink from the heater and no further heat can be applied to the ink. The bubble expands until all the excess heat stored in the ink is dissipated or used to vaporize liquid. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor, and the droplet is detached from the nozzle and propelled at a high speed in a direction towards a recording medium. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, a new portion of ink is supplied from the plenum. Then the same cycle is repeated again to generate another bubble and so on. The whole cycle takes just a few microseconds. In operation, each resistor element is connected via a conductive trace to microprocessor, where current-carrying signals cause one or more selected elements to heat up. In a piezoelectric printhead, as used, for example, in Epson Stylus series of printers, a piezoelectric transducer that produces a pressure pulse is used instead of the heater to eject the ink from the nozzle. The typical drop volume is of the order of 10 pl and the ejecting frequency from 1 up to 8 kHz. Recent advances in the printhead design allow one to achieve resolutions over 2400 dpi. Printheads designed to deliver so high resolution use nozzles with a diameter of less than 30 µm. It should be kept in mind that inks suitably employed in piezoelectric printers often cannot be used in thermal ink-jet printing, due to the effect of heating on the ink composition.

Component

Function

Water Water-miscible organic cosolvent Dye or pigment Binder Humectant Surfactant Biocide Buffer Other additives

Carrier medium Control wetting and drying characteristics Provide coloration Ensure fixation of colourant onto the surface Prevent crusting of ink in the printhead Control spreading and penetration Suppress biological growth Control pH, stabilize pigment dispersion Defoamer, jetting aid, solubilizer, anti-cockle and anti-curl additives, etc.

Both dyes and pigments are used as colourants in inks for ink-jet printing. In dye-based inks, the colourant exists in a form of a molecular solution, in a solubilized form, or in a form a latex dispersion or polymeric microemulsion impregnated with a dye. The majority of water-soluble dyes are ionic compounds. In pigment-based inks, the colourant exists as discrete particles. Water-based pigmented inks are prepared by incorporating the pigment in the continuous water phase by a milling and dispersing process. Pigmented inks require a water soluble dispersant in the pigment slurry during the milling process. Such a dispersant is necessary to produce a colloidally stable mixture and an ink that can be jetted reliably without clogging the print head nozzles. In general, as compared to pigment-based inks, dye-based inks deliver a superior colour quality, expansive colour gamuts and colour vibrancy leading to perfection in hue and shade reproduction. Also, the mechanics of printing dye-based inks via thermal ink jet are often simpler than those of printing pigment-based inks since pigments are colloidal objects, as opposed to the molecules of dyes. On the other hand, the image produced with water-soluble dyes has poor water resistance. If recording is carried out on plain paper, recording quality may deteriorate remarkably because of colour bleeding. Besides, dye-based inks have poor light resistance. Conversely, pigment-based inks may sometimes be plagued by pigment flocculation, agglomeration, and settling. Nowadays, water-based formulations are getting increasingly common. Water-based inks have advantages of ease of production of the ink, superior preservation stability, good colour hue

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20

and high colour density. In current best practice inks, water is used in combination with an appropriate co-solvent, such as ethylene glycol, butanediol, glycerol, propylene glycol laurate, ethylene glycol ethers, long chain alcohols, lactams including N-methylpyrrolidone and N-(2-hydroxyethyl)pyrrolidone, cyclic acetals and ketals, their polyoxyalkylenated derivatives, and the like. Some water miscible organic solvents that are listed above, such as ethylene glycol and diethylene glycol, may also act as surfactants. For example, isopropanol enhances spreading of the water component of the ink on any hydrophobic surfaces of the recording media for uniform dot formation. It was also reported about the use of sulphur-containing penetrants including alkyl sulphoxides R-SO2-R' and sulphones R-SO-R' [17] . The use of water as the major solvent has both advantages and disadvantages. Perhaps, the major advantage is the minimized environmental impact and fire hazard. On the other hand, the water-based inks can cause certain problems, the most common of which is the paper wicking followed by cockle and curl on drying. Besides, unless some special measures are taken, images printed with water-based inks will normally demonstrate inferior waterfastness as compared to those printed with solvent-based inks. Inks designed for printing on highly sized paper grades normally contain surfactants that promote wetting and penetration. However, this will potentially compromise the water-resistance of the printed stock, unless the surfactant is somehow deactivated after the ink is set up. Under especially unfavourable conditions, surfactants may enhance the toxicity of other chemical components and exacerbate puddling of the ink on the orifice plate. An undesirable reduction in edge acuity may also result from the addition of surfactants. Ink-jet inks are normally required to have a reduced surface tension in order to promote ink absorption, to minimize intercolour bleed, and to improve certain quality parameters, such as solid area uniformity and drying time. The major performance characteristics that determine the ink quality are:

the nozzle, thus reducing the number of misdirected drops7 [18]. By balancing the surface tensions of primary colour inks, such as cyan, magenta, and yellow, surfactants may help to minimize intercolour bleed and mottle. It has been reported that, in some cases, surfactants also may help to prolong the lifetime of the printhead by suppressing the formation of deposits on the heater surface, known as kogation, and reducing the risk of nozzle clogging. Phosphate esters are especially suitable for this purpose. Other common anti-kogation additives are polyphosphates, complexons, and hydroxyamide derivatives, but they do not have a surfactant effect. Ethylan CD® (Harcros Chemicals) and Peregal® O (GAF) surfactants comprising alcohol ethoxylates, Antarox® CO (GAF) surfactants comprising alkyl phenol ethoxylates, Nissan Nonion® (Nippon Oils & Fats), MAPEG® 1000 MS (PPG Specialty Chemicals) and Radiasurf® (Oleofina SA) surfactants comprising fatty acid esters of higher alcohols, Tergitol® (Union Carbide) surfactants comprising secondary alcohol ethoxylates, such as C12H25O(CH2CH2O)nH, Surfaron® A1500 (Protex Chemie) and Nekal® BA77 (GAF) surfactants comprising alkyl aryl sulphonates, Ethoxamine® SF (Witco Chemical SA) surfactants comprising tallow amine ethoxylates, as well as many other analogous products, are effective as wetters [19-21]. One can apparently use long-chain alcohols including octanol, undecanol, stearyl alcohol, 1,2-dodecanediol, 1,12-dodecanediol, etc., for the same purpose [22]. Major groups of surfactants that enhance wetting are summarized in Table 2. There is some evidence that hydrotropic amphiphiles, such as sodium benzoate, sodium salicylate, sodium benzene sulphonate, p-amino benzoic acid hydrochloride, resorcinol and pyrogallol, yield inks with faster dry times and improved bleed without the potential for puddling on the nozzle plate or the loss of edge acuity that sometimes plague inks containing conventional penetration-imparting surfactants. Hydrotropic amphiphiles, like other surfactants in general, can be used to solubilize an insoluble organic colourant in the ink without causing a precipitous drop in surface tension associated with the use of conventional surfactants [23,24]. Aerosol® OT, AY and GP (Cytec) surfactants comprising alkyl sulphosuccinates, Lomar® (Henkel Australia) surfactants comprising naphthalene formaldehyde sulphonate, Surfaron® (Protex Chemie) surfactants comprising sodium dioctyl sulphosuccinate, lauryl ether sulphate and a commercial line of Berol® 475 (Berol Nobel AB) surfactants comprising sodium alkyl ether sulphates, are used as pigment dispersants [25]. Ethoxylated monoalkyl- and dialkylphenols, such as Igepal® CA and CO (Rhone-Poulenc), Tamol® (Rhom and Hass), Dowfax® (Dow Chemical Europe), Triton® (Union Carbide), are also useful as dispersants. Small amounts of Dowfax® surfactants help to inhibit corrosion of the nickel orifice plate in thermal printheads [26]. These nonionic dispersants can be used alone or in combination with the above-mentioned anionic dispersants. Silicon surfactants such as Agitan® (Munzig Chemie), Mykon® DF (Warwick International), Moussex® (Protex Chemie) etc., are conventional defoamers [25]. The amount of the surfactant is usually within a range of from 0.01 to 5 % by weight based on the total weight of the ink. Since referring to fancy but often meaningless trade names of commercially available stuff would be of little help for the reader's understanding of why and how specific ink properties are influenced by a given surfactant, Table 2 provides more specific information about the chemical identity of various groups of surfactants proffered for use in inkjet inks [18-21,27-72].

• • • • • •

the rheological properties of the ink should match the discharging conditions of the printhead to ensure robustness with respect to both continuous and intermittent printing; the ink should exhibit low foaming; the ink should be readily fixable onto recording media, should dry sufficiently fast; the printed image should be of high quality and should exhibit good water- and lightresistance), smear resistance, and scratch resistance; the ink formulation should not chemically inert and nontoxic; the ink should have good stability towards long term storage over a sufficiently broad temperature range.

Surfactants, along with other additives, are needed to adjust some specific ink properties. Usually, surfactants are used as: (i) (ii) (iii)

wetters; emulsifiers and dispersants; and antifoamers.

Surfactants used in ink jet inks should have sufficiently high solubility in water in combination with a high surface activity. By reducing the surface tension of inks, surfactants both impart a high wettability to the ink and destabilize undesired ink bubble formation in the firing chambers in the printhead. This action has been observed to minimize puddling of the ink on the surface of 7

It is believed that such faults as drop misdirection and missing nozzles may be caused, at least in part, by uneven wetting of the nozzle plate by the ink.

21

TABLE 2 Types of surfactants suitable for use in ink-jet inks Desired function

Appropriate groups of surfactants

wetting/penetration

acetylenic surfactants such as 3,6-dimethyl-4-octyne-3,6-diol and their ethoxylated analogs; alkyl- and alkylaryl sulphonates; alkyl sulphosuccinates; fluorinated surfactants; adducts of poly(oxyalkylene glycol) and fatty acids, fatty alcohols, fatty amines, sorbitan esters, alkanol amides, castor oil; poly(dialkyl-siloxanes); fatty imidazolines; sulphonated fatty esters; phosphated fatty esters; fatty amines and their derivatives; quaternary alkosulphate compounds; poly(propylene oxide)/poly(ethylene oxide) copolymers; alkyl sulphoxides and alkyl sulphones; poly(alkylene glycol); carboxymethylamylose anionic: sodium alkyl sulphates, sodium dodecylbenzene sulphonate, sodium dodecyl naphthalene sulphate, sodium dodecyl diphenyloxide disulphonate, sodium alkyl sulphosuccinates, potassium N-methyl-N-oleoyl taurate; cationic: dialkyl benzenealkyl ammonium chloride, alkylbenzyl methyl ammonium chloride, cetyl pyridinium bromide, alkyl trimethyl ammonium bromides, halide salts of quaternized polyoxyethylalkylamines, dodecylbenzyl triethyl ammonium chloride; non-ionic: polyvinyl alcohol, polyacrylic acid, hydrophobically-substituted polyacryl amide, methyl cellulose, ethyl cellulose, hydroxy ethyl cellulose, carboxy methyl cellulose, polyoxyethylene alkyl ethers, polyoxyethylene nonylphenyl ether, alkyl or dialkyl phenoxy poly(ethyleneoxy)ethanol derivatives silicon compounds, blends of organic esters in mineral oil base, EO/PO block copolymers

dispersion/emulsification

defoaming

It should be noted that, because the mechanism whereby surfactants enhance wetting and stabilize disperse phases is the same, viz.: a reduction in the excess free energy associated with the interfacial tension, many of the listed surfactants are multipurpose surfactants in a sense that a wetter will enhance the stability of a lyophobic dispersion, and a dispersant may as well promote wetting. As a rule of thumb, polymeric surfactants should be preferred as dispersants because, apart from thermodynamic stabilization, they also provide steric stabilization of disperse phases. Conversely, low-molecular weight surfactants with a high cmc value should be preferred as wetters because they demonstrate fast surface tension relaxation. Certain compatibility issues should be kept in mind when deciding which surfactant is appropriate for a given formulation. In particular, if an cationic dye is used, one should avoid using anionic surfactants, and vice versa, an anionic dye should not be mixed with a cationic surfactant. Otherwise, dye precipitation by surfactant may occur. For example, it has been shown that Duoquat®T50 (Akzo Chemie) surfactant comprising quaternary alkyl diamine of the formula [C18H37-N(CH3)2-(CH2)3-N(CH3)3]Cl2 precipitates anionic dyes such as Bernacid Red 2BMN [73]. On the other hand, anionic surfactants can interact with cationic mordants8 sometimes used in the composition of ink receptive layers to immobilize acidic dyes. Apart from these common-use surfactants, specialty surfactants are sometimes needed to address some specific performance issues. For example, as already mentioned before, surfactants can be used to reduce intercolour bleed [74-77]. Usually, low HLB surfactants are employed for 8 A mordant is a compound that is included into the composition of ink-receptive coatings in order to enable dye fixation. For example, water-soluble high-molecular weight polyamines, such as poly(4-vinylpyridine), enable rapid fixation of acidic dyes.

22 this purpose. Examples of surfactants that proved to be particularly effective as bleed-alleviating additives in ink formulations include

CH3

CH3 + N O

C12H25

CH3

C12H25

N

O CH2CH2O

P

OR

OH phosphate esters of ethanol amine

N-alkyl amine oxides

CH3 C12H25

N

CH2COOH

CH3 N-alkyl glycines

CH3 C12H25

+ N (CH2)3SO3 CH3

alkyl sulphobetaines

Examples of other surfactants that are successfully employed to control bleed in ink-jet ink compositions are N,N-dimethyl-N-tetradecyl amine oxide; N,N-dimethyl-N-hexadecyl amine oxide; N,N-dimethyl-N-octadecyl amine oxide; and N,N-dimethyl-N-(9-octadecenyl) amine oxide [77]. These could be used in combination with appropriate co-surfactants including secondary alcohol ethoxylates which are helpful in improving the wetting and cloud point characteristics of low HLB surfactants [78]. Incorporation of dyes into micelles is believed to be a possible mechanism by which surfactants control bleed. As mentioned earlier in relation to the surface tension dynamics, diffusion of micelles is slower than that of disaggregated surfactant, and hence, mobility of solubilized dyestuff is limited. Micelles with dye of one colour will not exchange dye of another colour in an adjacent micelle on paper medium, provided that the ink setting time is much shorter than the time required for the solubilized dye to diffuse through the micellar medium. The efficiency of this bleed alleviation depends upon the level of adsorption of the dyes into the micelles, the amount of micelles in the ink, and the diffusivity of dye and micelles on the paper surface. In the case of a poorly solubilized dye, a much higher surfactant concentration is necessary to bind the same amount of dye than in the case of a better solubilized one. The propensity for dye to get solubilized is determined by the structural characteristics of the dye molecule, the surfactant, and the solvent. It has also been described the use of amphiphilic dyes which combine the dyeing and surface active functions in one molecule [79]. Adducts of the hydroxyamide derivatives with ethyleneoxide or propyleneoxide, including N-(2-hydroxyethyl)butyric acid amide, N-imidazoyl-N-(2-hydroxyethyl)acetic acid amide, N-butyl-N-(2-hydroxyethyl)propionic acid amide, N-pyridyl-N-(2-hydroxyethyl)acetic acid amide, and the like, proved to be effective as anti-curl additives [80]. Dihydroxysulphone derivatives of general formula HO-(CH2CH2O)n-(CH2)m-SO-(CH2)n'-(OCH2CH2)m'-OH, including 2-hydroxyethyl-l,3'-hydroxypropyl sulphone, 3-hydroxypropy-l,4'-hydroxybutyl sulphone, bis(2hydroxyethyl)sulphone, bis(3-hydroxypropyl)sulphone, bis(4-hydroxybutyl)sulphone, and their analogs, can be used for the same purpose [81].

23

24

8. Selected groups of performance surfactants Gemini surfactants The molecule of gemini surfactants has two moieties joined together by means of the so-called spacer. Each moiety has its own hydrophobic and hydrophilic parts, so that the whole molecule looks like a symmetric couple. Some examples of gemini surfactants are shown below:

CH3 (CH3)2CHCH2

C

CH3 C

C

C

CH2CH(CH3)2

(OCH2CH2)nOH (OCH2CH2)nOH Ethoxylated acetylene glycols, e.g. Surfynols® surfactants

C12H25

CH3

CH3 + N (CH2)n

N

CH3

CH3

+ C H 12 25

dimeric alkyl ammonium salts

O

SO3Na

C10H21O Y C10H21O O

SO3Na

Y = -O-, -OCH2CH2O-, -O(CH2CH2O)ndimeric sulphonated polyoxyethylene alkyl ethers

In gemini surfactants, both hydrophilic groups are normally situated in close vicinity to the spacer whereas the hydrophobic parts are situated at the ends of the molecule.9 Such a unique structure reduces the entropy loss related to the confinement of surfactant during adsorption. This effect is analogous the chelating effect in complexation and accounts for high surface activity of hemini surfactants.

9 This discerns gemini surfactants from so-called bolaform surfactants in which two hydrophilic groups are situated at the ends of a hydrophobic chain.

The surface activity, adsorption kinetics, and aggregation properties of geminis depend upon the length of the spacer or the branchiness of the aliphatic tail. Because of their high surface activity, gemini surfactants demonstrate exceptional wetting characteristics, and in some cases, resist re-solubilisation of dye or pigment after the ink has set up. Furthermore, because gemini surfactant can displace other film-forming surface-active compounds from the liquid/vapour, they can be used as mild de-foamers. It has also been reported that gemini surfactants can be used as anti-pinhole additives. Many geminis are available commercially. Surfynol® (Air Products) surfactants, which comprise adducts of 2,4,7,9,tetramethyl-5-decyne-4,7-diol and polyoxyethylene glycol, were claimed to be effective as wetting and bleed-alleviating agents for ink-jet inks [61-67]. Acetylenol® EH (Kawaken Fine Chemicals Co., Ltd) surfactant was used as wetter [69]. An example of ink-jet ink formulation containing a Surfynol® surfactant is: 0.68% water soluble dye, 0.5% Surfynol® TM 465, 6% glycerol, 6% diethylene glycol, and the rest is water [64]. By combining gemini surfactants with conventional ionic or nonionic amphiphiles, a variety of synergetic effects can be exploited. Fluorinated surfactants Because of exceptional chemical stability of fluorocarbon residues, fluorinated surfactants are resistant to extreme temperature conditions and aggressive environment. The compactness of fluorocarbon group favours formation of compact adsorbed layers and enables micellization of relatively short-chain surfactants. For example, perfluoroalkanoic acids exhibit surfactant properties and form micelles already when the chain length is four carbon atoms, whereas regular fatty acids start to show their surface-active properties when the alkyl chain contains at least eight carbon atoms. Unlike many traditional surfactants, fluorinated surfactants preserve their surfaceactive properties in non-aqueous solutions. Fluorosurfactants improve the wetting characteristics of water/glycol inks for plain paper without drying of the ink-jet nozzles. They can also be used as wetters in ink formulations designed for printing on low-energy surfaces such as plastic films and the like [38-42]. At the same time, they behave as dewetting agents for high-energy surfaces. In general, fluorosurfactants promote foaming, which limits their use in inks [41], although some anionic surfactants of this class proved to act as antifoaming agents [37]. Many fluorosurfactants, such as Fluorad® (3M), Zonyl® (DuPont), are available commercially. The major disadvantages of fluorinated surfactants are their high price and poor biodegradability. Examples of fluorinated surfactants suitable for use in ink-jet inks are F(CF2CF2)3-8CH2CH2PO4(NH4)2, F(CF2CF2)3-8CH2CH2SCH2CH2COOLi, F(CF2CF2)3-8CH2CH2(OCH2CH2)1-10OH [41]. Another group of fluorosurfactants suitable for use in ink-jet inks are anionic bitail fluorothioalkyl surfactants [73]. A typical member of this group of surfactants is (C10F21-CH2-S)2C(CH3)CH2CH2COOLi available commercially as Lodyne P200 (Ciba-Geigy). An interesting feature of this surfactant is its ability to form bilayers at the liquid/vapour interface. To give the reader a feeling what an actual ink may look like, the ingredients of one patented formulation [38] are listed in Table 3.

25

26

TABLE 3 Example of ink-jet ink formulation containing a fluorosurfactant Ingredient Cyan pigment dispersion concentrate* Magenta pigment dispersion concentrate* Yellow pigment dispersion concentrate* Black pigment dispersion concentrate* Emulsion Polymer E1691 Tetomer B mono ether with PE glycol Polyether-modified dimethyl polysiloxane Water

Amount (wt.%) 30 18 2 2 48

30 18 2 2 48

30 18 2 2 48

30 18 2 2 48

*According to the cited invention, the pigment concentrates contain appropriate pigments, acrylic polymer and diethylene glycol mixed in a ratio around 5:3:1 by weight.

It should be noted, however, that practical ink formulations are rarely disclosed by ink manufacturers. Hence, the above example most likely represents no more than a pedagogic interest. Polymerizable surfactants It is always a challenging task to develop a surfactant formulation which would enable wetting of the printed substrate at the moment the ink is applied but prevent subsequent rewetting of the print under the action of moisture. For example, alkoxylated fatty acid alkanolamides can react with alkyd binder, which reduces the tendency of surfactant to migrate to the surface of the ink film rendering this more sensitive to moisture. Major developments in this area concern surfactants that can be used in emulsion polymerization. Polymerizable surfactants find some limited application in the coatings technology. As to practical ink formulations, polymeric surfactants or reactive dyes seem to be a preferable option. The use of polymers derived from unsaturated surfactants unexpectedly improves several properties of ink-jet inks, whether the colourant is a water-insoluble pigment or a water-miscible dye [47,82]. Polymeric surfactants Polymeric surfactants are a class of high-performance products that can be used to assist emulsification and dispersion of dyes and pigments. Due to a large number of active sites per molecule, the polymeric surfactants demonstrate high adsorption affinity to a variety of surfaces and provide effective steric stabilization of dispersed phases both in aqueous and non-aqueous environment. By knitting surfactant in a polymeric structure, independent movement of the surfactant in the ink is prevented. Therefore, the negative effects associated with excessive mobility of unbound surfactant, such as print-through caused by excessive substrate penetration, and loss of edge acuity and intercolour bleed caused by Marangoni flow, can be avoided. Lack of independent movement, on the other hand, would not eliminate the positive aspects provided by a surfactant, such as increased pigment wetting and decreased drying time [47,49]. It has also been observed that the presence of some polymeric surfactants to inks can improve drop formation characteristics [50]. The most common types of polymeric surfactants are alkoxylated polyalcohols, polyesters produced by reacting aliphatic carboxylic acids or polycarboxylic acids with polyethylene glycol, modified celluloses such as ethyl(hydroxyethyl)cellulose, polyoxyethylated

polyoxypropylene glycols and other ethylene oxide/propylene oxide copolymers and block copolymers; many are available commercially. As an example, the Dapral® line of comb copolymers manufactured by Akzo Chemicals can be mentioned. These are especially suitable to stabilize polar disperse phases in lower polarity vehicles. Pluronic PE® (BASF), Radiasurf® (Oleofina SA), and Synperonic® (ICI Chemicals) surfactants representing ethylene oxide/propylene oxide copolymers can be used as defoamers in water-based inks. Some polymerics surfactants, e.g. an acrylic resin commercially available as Joncryl® 61LV (S. C. Johnson Polymer), combine dye-coupling and dispersive functions and can be used to produce water-fast print [70]. There have been also a few reports about the use of carboxylated lignin in a function of binder in ink-jet inks [44]. The application of hydrophobically-substituted oligomeric acrylamide dispersants of the type,

C12H25S

[CHCH2]10 H CONH2

C18H37S

[CHCH2]4 H CONHC(CH3)2CH2SO3Na

and the like, which proved to be useful with a wide variety of pigments in inks, is disclosed in [66]. Ionic dispersants are produced by polymerization of appropriate unsaturated compounds containing ionic water-dispersing groups. These include acrylic acid and its derivatives, maleic acid and its derivatives, citraconic acid, styrenesulphonic acid, vinylbenzylsulphonic acid, vinylsulphonic acid, acryloyloxyalkyl sulphonic acids, mono-(acryloyloxyalkyl) phosphates and so on. Non-ionic dispersants are produced by polymerization of appropriate unsaturated compounds, such as alkoxy polyethylene glycol methacrylates, which contain non-ionic waterdispersing groups. The above-mentioned compounds can also be co-polymerized with other olefinically unsaturated monomers, such as alkyl methacrylates, substituted styrenes, methacrylamides, allyl compounds, dienes, vinyl ethers, vinyl ketones, vinyl halides, vinylidene halides, olefins, unsaturated nitriles, etc., which do not have any polar groups. In this way, the ratio of hydrophobic and hydrophilic functionalities in the polymeric surfactant can be controlled [47,48]. Silicon surfactants Silicone surfactants are built around a polydimethylsiloxane backbone to which different hydrophilic groups, such as polyoxyethylene glycol, can be attached. Siloxane surfactants are characterized by high chemical and thermal stability, and reduce the surface tension of aqueous solutions to 20 mN m-1. At the same time, because of a high adsorption affinity of siloxane surfactants to hydrophobic surfaces, the surface tension of the solid/liquid may even become negative, thus yielding a positive value for the spreading coefficient (eq.34). Within a homologues series, surfactants forming vesicles better promote spreading than those forming micellar solutions. The best wetting properties have short-chain siloxane hydrophobes containing 2 to 5 silicon atoms. Unfortunately, because of hydrolytical instability of the siloxane bond, water-soluble siloxanes undergo slow hydrolysis in an aquatic environment. The polymethylsiloxane moiety is not only strongly hydrophobic but also lipophobic with respect to a variety of non-polar solvents. This makes it possible to synthesize alkyl or alkyl phenyl substituted structure which are surface active in organic solvents. Siloxane surfactants can be used, sometimes in combination with other surfactants, in water-based ink-jet inks designed for printing on low-energy surfaces, such as vinyls and plastics

27

28

[18,38,41,58-60,68,83]. As an example of surfactants suitable for this purpose, one can mention polyoxyethylene-grafted siloxanes and siloxane/polyoxyethylene copolymers, e.g.

CH3 CH3 Si CH3

CH3

CH3 Si

CH3

N

CH3

O Si O

(CH2)3

Si m

N R

CH3

Convenient synthetic methods for the preparation of quaternary, alkoxy, alkylated, phosphonated, sulphonated and carboxylated derivatives have been developed. A commercial product sold as Imidazoline 180H (Lakeland Laboratories) is effective as wetter and pigment dispersant.

CH3 Polyamines

(OCH2CH2)nOH [O-Si(CH3)2-]m [OCH2CH2-]n OCH3 CH3Si

[O-Si(CH3)2-]m [OCH2CH2-]n OCH3 [O-Si(CH3)2-]m [OCH2CH2-]n OCH3

Some are available commercially under the Silwet® (Witco Chemicals) trade mark [25,59,60]. So-called M(D'EnOH)M superspreaders with a formula ((CH3)3SiO)2Si(CH3)-(CH2)2-(OCH2CH2)-OH are manufactured by Dow Corning. It should also be noted that siloxane surfactants have a long history of use as defoaming agents [25]. Polymeric alkoxysilanes, e.g. various polymers and co-polymers of 3(trimethoxy)silylpropyl methacrylate and their analogs, are also used as binders [83]. Alkyloxypropylamines Alkyloxypropylamines of general formula CnH2n+1O-(CH2)3-NH2 have been used to modify the dispersion characteristics of dyestuff and improve solubility of dyestuff in oil-based systems. Various linear and branched alkyloxypropylamines have been used in the synthesis of phthalocyanine and azoic dyes for use in water-based ink-jet ink formulations [84,85]. Phospholipides Most of the phospholipides are derivatives of glycerine as hydrophilic component and are also designated as phosphoglycerides or phosphatides. Phospholipides are amphiphilic compounds and demonstrate properties typical of surfactants. In particular, phospholipid molecules can aggregate forming vesicles − mostly spherical structures with a double membrane with the lyophobic portions of the molecules pointing toward each other. It has been shown that vesicles can encapsulate pigment particles, thus providing steric stabilization of pigment dispersions. There have been communications about the use of phospholipides as pigment stabilizers in solvent-based ink-jet inks [86-88]. 2-Alkyl Imidazolines These surfactants hold a rather unique place among cationic amphiphiles, for they combine excellent wetting and dispersing properties with a broad spectrum of antibacterial and antifungal activity, outperforming many more conventional biocides such as benzoate or sorbate salts. The general formula of 2-alkyl imidazoline is

It has been reported that sulphated polyalkylenpolyamines can be used in combination with polyphosphates to prepare water-fast inks suitable for printing onto plain paper. Examples of such CnH2n+1(NHC2H4)NHC2H4SO3Li, compounds are CnH2n+1(NHC2H4)2NHCH2COONa, CnH2n+1(NHC2H4)NHCH2SO3H⋅N(C2H5)3, CnH2n+1N(CH3)C2H4N(CH3)C2H4NHC2H4COONa, etc. [89]. Acknowledgement

This work was supported by the Papers Surfaces for Digital Printing (S2P2) Programme and the Engineering and Physical Sciences Research Council (EPSRC). References

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